zipfs: fix "got block count 8, want 1" test failure

Failure was:

    zipfs_test.go:74: got block count 8, want 1

This got broken by "fs: set Blksize and Blocks automatically based on Size",
fix it by setting Blksize.

Change-Id: I1d3798082b2e8a7fcda908b9b88d8b51fb11da78

zipfs/zipfs.go

@@ -84,7 +84,9 @@ func (zf *zipFile) Getattr(ctx context.Context, f fs.FileHandle, out *fuse.AttrO
 	out.Atime = out.Mtime
 	out.Ctime = out.Mtime
 	out.Size = zf.file.UncompressedSize64
-	out.Blocks = (out.Size + 511) / 512
+	const bs = 512
+	out.Blksize = bs
+	out.Blocks = (out.Size + bs - 1) / bs
 	return 0
 }